This invention relates generally to inertial navigation systems which require gravity vector data in their operational regions and more specifically to methods for determining the deflection of the gravity vector at a point above the earth's surface given gravity vector deflection data for a three-dimensional grid of points above the earth's surface.
World wide deflection of vertical (DOV) data provided by the National Imagery and Mapping Agency (NIMA) can be used to compensate the mismodeled gravity vector error, thereby reducing velocity and position errors from an inertial navigation system.
NIMA provides DOV data on 32 CD-ROMs with each CD/ROM containing a 45.degree..times.45.degree. sector of data. DOV data (East-West, North-South, as well as associated accuracies) are stored in files corresponding to grid points that are 2 arcminutes apart in 1.degree..times.1.degree. cells.
The future NIMA DOV database will consist of 180.times.360+1=64,801 files at one altitude. There will be seven reference altitudes at 0, 10K, 20K, 30K, 50K, 70K, and 90K feet. This represents the NIMA-supplied data for the F-117 aircraft.
Each of the 64,801 files (except for the one discussed in the next paragraph) contains DOV data and accuracy data (all in arcseconds) of 900 grid points at 2 arcminutes spacing in a 1.degree..times.1.degree. angular cell as shown in FIG. 1. The files are labeled from N01000, N01001, . . . , N01359, N02000, . . . , to N90359 and S0000, S00001, . . . , S00359, S01000, . . . , to S89359. Each file is 29,025 bytes in size.
The remaining file is named S90000. It contains the DOV and accuracy data for the south pole repeated 900 times to maintain the uniformity of data files.